1. Field of the Invention
The present invention relates to a device for detecting the deterioration of a catalytic converter on an internal combustion engine, and more particularly, relates to a device which is capable of detecting the deterioration of a catalytic converter disposed in the common exhaust passage of an engine having cylinders divided into multiple-cylinder groups such as a V-type or horizontally-opposed engine.
2. Description of the Related Art
An air-fuel ratio control device for controlling the air-fuel ratio of an engine by feedback control based on an output signal of an O.sub.2 sensor, which generates a voltage signal depending on whether an air-fuel ratio of the exhaust gas is on rich side or lean side compared to a stoichiometric air-fuel ratio, disposed in an exhaust passage upstream of a catalytic converter is known as a single O.sub.2 sensor system. The single O.sub.2 sensor system is used to control the air-fuel ratio of the engine at a stoichiometric air-fuel ratio and to reduce polluting exhaust emissions by utilizing the capacity of a three-way catalytic converter to the maximum.
Also, to compensate for individual differences in the output characteristics of the O.sub.2 sensors, or changes thereof due to aging, a double O.sub.2 sensor system using two O.sub.2 sensors has been developed (U.S. Pat. No. 4,739,614). In the double O.sub.2 sensor system, O.sub.2 sensors are disposed upstream and downstream of the catalytic converter in the exhaust passage, and the air-fuel ratio control is carried out based on the output of the downstream O.sub.2 sensor as well as the output of the upstream O.sub.2 sensor. According to the double O.sub.2 sensor system, since fluctuations in the output of the upstream O.sub.2 sensor are compensated for by a feedback control using the output of the downstream O.sub.2 sensor, good emission control characteristics are maintained even when the output characteristics of the upstream O.sub.2 sensor deteriorate.
Nevertheless, even in the double O.sub.2 sensor system, if the catalyst in the catalytic converter deteriorates, the ability of the catalyst to remove pollutants in the exhaust gas such as HC, CO, NO.sub.x falls. Therefore, it is necessary to detect deterioration of the catalyst in the catalytic converter accurately.
To detect deterioration of the catalyst in the catalytic converter used in the double O.sub.2 sensor system, various methods and devices have been proposed.
For example, the deterioration of the catalyst in the catalytic converter can be determined by detecting a deterioration in the O.sub.2 storage capacity of the catalyst. That is, the catalyst has an ability to adsorb oxygen from the exhaust gas when the air-fuel ratio is rich compared to the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio), and to release the adsorbed oxygen when the air-fuel ratio is lean compared to the stoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio). This capacity, i.e., the O.sub.2 storage capacity of the catalyst, falls as the deterioration of the catalyst proceeds. Therefore, when the catalyst is in normal condition, the fluctuation of the air-fuel ratio of the exhaust gas downstream of the catalytic converter is small, and consequently, the fluctuation of the output of the downstream O.sub.2 sensor also becomes small even if the air-fuel ratio of the exhaust gas is oscillates between a rich air-fuel ratio and a lean air-fuel ratio. On the contrary, if the catalyst has deteriorated, since the O.sub.2 storage capacity also becomes small, the air-fuel ratio of the exhaust gas downstream of the catalytic converter oscillates in a similar manner as the oscillation of the air-fuel ratio of the exhaust gas upstream of the catalytic converter, and the output of the downstream O.sub.2 sensor also fluctuates as the output of the upstream O.sub.2 sensor fluctuates. Therefore, when the catalyst has deteriorated, the interval between reversals of the output signal of the downstream O.sub.2 sensor (i.e., the period between changes of the output signal of the downstream O.sub.2 sensor from a rich air-fuel ratio signal to a lean air-fuel ratio signal, or vice versa ) during air-fuel ratio feedback control, becomes shorter (in other words, the number of the reversals of the output signal of the downstream O.sub.2 sensor within a predetermined time becomes larger), and the amplitude of the fluctuations in the output signal of the downstream O.sub.2 sensor becomes larger at the same time.
In the system disclosed in U.S. Pat. No. 4,739,614, it is determined that the catalyst has deteriorated when the ratio of the interval between the reversals of the upstream O.sub.2 sensor T.sub.1 to the interval between the reversals of the downstream O.sub.2 sensor T.sub.2, i.e., T.sub.1 /T.sub.2 becomes larger than a predetermined value (or, alternatively, when the interval T.sub.2 of the downstream O.sub.2 sensor becomes smaller than a predetermined value).
Also, a method for detecting deterioration of the catalyst is disclosed in the copending U.S. patent application Ser. No. 957041, in which lengths and areas of the waveforms of the output signal response curves of the upstream and the downstream O.sub.2 sensors are used for detecting deterioration of the catalyst. In this method, the lengths (LVOM and LVOS) of the output signal response curves of the upstream O.sub.2 sensor and downstream O.sub.2 sensor and, the areas (AVOM and AVOS) bounded by the response curves of the output signals of the upstream and the downstream O.sub.2 sensors and the reference voltage lines thereof, are used for determining whether or not the catalyst has deteriorated.
When the catalyst is not deteriorated, the area AVOS bounded by the output response curve of the downstream O.sub.2 sensor and the reference voltage line is relatively large although the length LVOS of the output signal response curve thereof is relatively small. On the other hand, the area AVOS becomes relatively small although the length LVOS becomes relatively large when the catalyst has deteriorated. In the method disclosed in the copending U.S. patent application Ser. No. 957041, the ratio of the above lengths (i.e., LVOS/LVOM) and the ratio of the above areas (i.e., AVOS/AVOM) are calculated and, the catalyst is determined as being deteriorated, for example, when the relationship between the ratios LVOS/LVOM and AVOS/AVOM fall in the hatched area in the map shown in FIG. 13. By this method, a deterioration of the catalyst can be detected accurately even if the O.sub.2 sensor itself is deteriorated to some degree.
The double O.sub.2 sensor system is also applied to engines, such as V-type or horizontally-opposed engines, in which the cylinders of the engine are divided into two or more cylinder groups. In this case, individual exhaust passages from the respective cylinder groups are merged into one common exhaust passage, and a catalytic converter is disposed in said common exhaust passage. The upstream O.sub.2 sensors, one for each cylinder group, are disposed in the respective individual exhaust passages, and single downstream O.sub.2 sensor is disposed in the common exhaust passage down stream of the catalytic converter.
An example of this type of multiple O.sub.2 sensor system (which is called "triple O.sub.2 sensor system") is disclosed in Japanese Unexamined Patent Publication KOKAI) No.1-8332. In this system, the air-fuel ratios of the respective cylinder groups are controlled independently from other cylinder groups based on the output signal of corresponding upstream O.sub.2 sensor. However, the respective air-fuel ratio controls of the cylinder groups are corrected based on the output of the common downstream O.sub.2 sensor.
However, in the triple O.sub.2 sensor system, it is difficult to detect the deterioration of the catalytic converter based on the output of the downstream O.sub.2 sensor. In the triple O.sub.2 sensor system, since the air-fuel ratios of the respective cylinder groups are controlled independently based on the output of the corresponding upstream O.sub.2 sensors, the air-fuel ratios of the respective cylinder groups do not change in a synchronous manner. In other word, frequencies, amplitudes, or phases etc. of the oscillation in the air-fuel ratios of the exhaust gases from the respective cylinder groups are different. These exhaust gases having different characteristics of oscillation of the air-fuel ratio, flow into the common exhaust passage and are mixed with each other before flowing into the catalytic converter. Consequently, the frequency, amplitude, or center value etc., of the oscillation of the air-fuel ratio of the exhaust gas flowing into the catalytic converter does not coincide with those in the oscillation of the air-fuel ratio of any of the cylinder groups.
For example, even if the air-fuel ratio of the exhaust gas in each individual exhaust passage oscillates regularly between a rich air-fuel ratio and a lean air-fuel ratio, the air-fuel ratio of the exhaust gas after mixed in the common exhaust passage may stay on either a rich air-fuel ratio or a lean air-fuel ratio, and may not change between a rich air-fuel ratio and a lean air-fuel ratio, or, in some cases, the amplitude of the oscillation of the air-fuel ratio of the exhaust gas after mixing may become very small.
In such cases, the amplitude of the oscillation of the output become very small, or in some cases, the reversal of the output signal of the downstream O.sub.2 sensor does not occur even if the catalyst has deteriorated. Therefore, it is very difficult to detect a deterioration of the catalyst based on the number (or the period) of the reversals of the output signal of the downstream O.sub.2 sensor. In such cases, since none of the output signals of the upstream O.sub.2 sensors coincide with the air-fuel ratio of the exhaust gas flowing into the catalytic converter, it is also difficult to detect a deterioration of the catalyst based on the lengths and areas of the output signal response curves of the upstream and the downstream O.sub.2 sensor using the method disclosed in the copending U.S. patent application Ser. No. 957041.
To solve the above-mentioned problem, U.S. Pat. No. 5,207,057, proposes a device for detecting the deterioration of a catalytic converter used in the triple O.sub.2 sensor system, in which the individual air-fuel ratio control of the respective cylinder groups based on the respective upstream O.sub.2 sensors is stopped when the detection of the deterioration of the catalytic converter is carried out. That is, when the detection of the deterioration of the catalytic converter is to be carried out, the device controls the air-fuel ratios of all the cylinder groups based on one of the upstream O.sub.2 sensors, so that the air-fuel ratios of all the cylinder groups change synchronously. Thus, the changes in the air-fuel ratio of the exhaust gas flowing into the catalytic converter after mixing also becomes synchronous with the changes in the air-fuel ratio of the exhaust gases before mixing, and deterioration of the catalytic converter can be detected effectively using the output of the downstream O.sub.2 sensor.
However, in the above system, the air-fuel ratio controls of the respective cylinder groups are forced to switch from the individual control to a common control so that air-fuel ratio control of each cylinder group is synchronized to one particular cylinder group. Therefore, when the air-fuel ratio feedback control is switched, the period of the air-fuel ratio feedback control of the switched cylinder groups becomes longer than usual during the period of switching. This may cause deterioration of the control characteristics and temporary deterioration of exhaust emissions.